1. Field of the Invention
The present invention relates to a solder alloy, and a soldered bond of an electric or electronic device, that has improved environmental safety.
2. Description of the Related Art
Lead-tin (Pbxe2x80x94Sn) solder alloys have conventionally been used in a large amount for solder bonding of various kinds of electric and electronic devices because they have a low melting point and good wettability even in oxidizing atmosphere such as air. However, because of the toxicity of Pb, regulations have been imposed on the services and operations using Pb and Pb-containing alloys, so that the occurrence of Pb-intoxation has been heretofore minimized.
Recent increase in the concern about environmental disruption is raising questions about the disposal of various industrial devices using Pb-containing solder alloys, such as electric and electronic devices.
The electric and electronic devices have been generally disposed of by reclamation as in the case of usual industrial and nonindustrial wastes. However, concern about the impact of harmful effect on environment and living organism, caused by elution of Pb if a large amount of electric or electronic device wastes including Pb-containing solder alloys, is kept being disposed by reclamation without pretreatment.
In the near future, it will probably be made compulsory to collect Pb from used electric and electronic devices, which include a large amount of Pb-containing solder alloy, before disposal.
However, it is not yet technologically possible to remove Pb efficiently and effectively from the used electric and electronic devices. It is also possible that the Pb collection cost would cause an increase in the product cost.
Therefore, it is strongly desired that a Pb-free solder alloy containing no Pb is developed.
As a Pb-free solder alloy, a Sn-based alloy containing combined additives such as Zn, Ag, Bi and Cu has been put into practical use, but only in limited special applications. This is because the alloy does not have good solderability, i.e. the essential properties of the conventional Pbxe2x80x94Sn solder alloy for general applications such as low melting point, good wettability, capability for reflow treatment, freedom from reaction with the base material that would cause formation of a brittle compound layer or an embrittled layer.
Snxe2x80x94Zn solder alloy is most useful Pb-free solder alloy at present. Snxe2x80x94Zn solder alloy has a melting point of near 200xc2x0 C. and is expected to provide a replacement for the conventional Snxe2x80x94Pb alloy.
However, Zn easily oxidizes and has poor solder wettability and it is necessary to use nitrogen gas or other non-oxidizing atmospheres in order to ensure good solderability.
To provide Snxe2x80x94Zn solder alloy with an improved wettability, the addition of Cu or Ge was proposed but this did not provide an expected improvement in wettability. Instead, the Cu addition causes rapid formation of Cuxe2x80x94Zn intermetallic compound in the solder alloy to deteriorate the properties of the solder alloy.
Moreover, Zn has so high an activity that, when soldering is effected on a Cu base material, a thick layer of Cuxe2x80x94Zn intermetallic compound easily forms even with a small heat input to reduce the bond strength. In this case, the base material/solder interface presumably has a structure of Cu base/xcex2xe2x80x2- CuZn layer/xcex3- Cu5Zn8 layer/solder layer. A Cuxe2x80x94Zn intermetallic compound has a very weak bond strength at the interface with the solder and this causes exfoliation to easily occur. This cannot be avoided if the Cu base material is coated with Ni/Au, Pd or Pd/Au plating.
Therefore, Snxe2x80x94Zn solder alloy has not yet been put into practical use from the viewpoint of the reliability of electronic devices.
The object of the present invention is to provide a Pb-free solder alloy and a soldered bond using the same, in which the solder alloy has no harmful environmental effect but has a solderability comparable to that of the conventional Pbxe2x80x94Sn solder alloy.
To achieve the object according to the first aspect of the present invention, there is provided a solder alloy consisting of:
Zn: 3.0-14.0 wt %,
Al: 0.0020-0.0080 wt %, and
the balance of Sn and unavoidable impurities.
According to the first aspect, there is also provided a soldered bond of an electric or electronic device, said bond being composed of a solder alloy consisting of:
Zn: 3.0-14.0 wt %,
Al: 0.0020-0.0080 wt %, and
the balance of Sn and unavoidable impurities.
According to the second aspect of the present invention, there is provided a solder alloy consisting of:
Zn: 3.0-14.0 wt %,
Bi: 3.0-6.0 wt %,
Al: 0.0020-0.0100 wt %, and
the balance of Sn and unavoidable impurities.
According to the second aspect, there is also provided a soldered bond of an electric or electronic device, said bond being composed of a solder alloy consisting of:
Zn: 3.0-14.0 wt %,
Bi: 3.0-6.0 wt %,
Al: 0.0020-0.0100 wt %, and
the balance of Sn and unavoidable impurities.
The contents of the component elements are limited for the following reasons.
Zn: 3-14 wt % (1st and 2nd Aspects)
Zn is an essential component to provide a low melting point and improved wettability of the solder alloy. The Zn content of 3-14 wt % stably ensures good wettability. A Zn content of either less than 3 wt % or more than 14 wt % deteriorates the wettability.
Al: 0.0020-0.0080 wt % (1st Aspect, Bi Absent)
Al: 0.0020-0.0100 wt % (2nd Aspect, Bi Present)
Al suppresses oxidation of Snxe2x80x94Zn alloy to ensure good wettability. As heretofore described, Zn easily oxidizes to form an oxide film on the solder layer that intervenes between the solder layer and a base material and blocks wetting of the base material by the solder. To ensure the oxidation suppression effect, Al must be present in an amount of 0.0020 wt % or more. However, experiments have shown that an excessive amount of Al deteriorates the wettability. Therefore, the Al content must not be more than 0.0080 wt % for the alloy of the first aspect and must not be more than 0.0100 wt % for the alloy of the second aspect.
Bi: 3.0-6.0 wt % (2nd Aspect)
Bi further lowers the melting point and improves the wettability of the solder alloy. To ensure this effect, the Bi content must be 3.0 wt % or more. However, excessive amounts of Bi cause the solder to have an excessively high hardness causing embrittlement, so that a reliable soldered bond cannot be obtained. Therefore, the Bi content must not be more than 6.0 wt.
Solder alloys, particularly solder alloys for soldering of electric or electronic devices must have the following properties.
1) Soldering can be effected at a low temperature as close as possible to that of the conventional Snxe2x80x94Pb eutectic solder alloy. Specifically, the soldering temperature should not be far above 200xc2x0 C., preferably not higher than about 220xc2x0 C.
2) Good wetting with the base material is ensured.
3) No brittle intermetallic compounds and no embrittled layers are formed by reaction of the solder alloy with the base material.
4) The alloying elements form no oxides which cause occurrence of poor wetting, voids, bridges or other defects.
5) The solder alloy can be provided in a form suitable for working and feeding in soldering on mass production lines, for example, in a solder paste form, a solder ball form for BGA, etc.
The present invention provides a Znxe2x80x94Alxe2x80x94Sn solder alloy and a Znxe2x80x94Bixe2x80x94Alxe2x80x94Sn solder alloy not only having an improved environmental safety but also satisfying the above-summarized property requirements.